1. Field of the Invention
The present invention relates to an interferometer making use of the interference phenomenon of light, a displacement measuring apparatus, and an information recording or/and reproducing apparatus using it. The present invention is suitably applicable, particularly, to industrial microlength meters having the resolving power and accuracy of the submicron order, based on application of the interference phenomenon of light, and machine tools and assembling/adjusting equipment utilizing them.
2. Related Background Art
Gage interferometers using lasers have been employed commonly as high-accuracy length measuring systems in various devices ranging from research fields to industry. In general, such systems are required to yield absolute accuracy and gas lasers of stable wavelength are used as light sources thereof.
However, simple gage interferometers using semiconductor lasers are commercially available lately and are showing features of compactness and simplicity. These gage interferometers are constructed generally in the structure based on the principle of the Michelson interferometer, in which a laser beam is emitted from the main body of interferometer to the outside to illuminate a reflector attached to an object to be measured and in which the laser beam is guided back to the original path to undergo interference in the main body of the gage interferometer. A measurable range depends upon coherency of the laser beam and is approximately equal to the coherent length. The gage interferometers using the gas lasers and the single-mode semiconductor lasers normally have the coherent length of several meters or more.
Since these gage interferometers have been applied heretofore to various stages, etc., for FA equipment, it was impossible to apply them to high-resolution measurement of displacement in a microrange and of a microportion of the millimeter order because of the restraints on the size of apparatus. In these fields, for example, where objects to be measured were limited to metal, the displacement was measured using xe2x80x9celectrical capacitance sensorsxe2x80x9d making use of the phenomenon that electrical capacitance varies depending upon spacing between the measured object and the sensor body.
There was, however, a limit to compactification in order to yield high accuracy and high resolving power (high S/N), because the measured objects were metal and because the capacitance was associated with the size of the measured objects (the area of a detected surface). The capacitance sensors are unable to measure the capacitance in a complete contact state, but are able to provide best signals in a close proximity state. Therefore, they have to be adjusted in an almost contact state. This is not so preferable because there is the possibility of damaging the measured object. There are additional limitations to use conditions, for example, the condition in which accurate measurement is allowed only in a state near to a standstill state.
On the other hand, in the case of the gage interferometers using the semiconductor lasers as a light source, the light source itself can be realized in the size of the millimeter order, but the single-mode semiconductor lasers experience the mode hopping phenomenon of switching between oscillation modes depending upon operating environment variations, etc., and thus discontinuously change their interference phases every occurrence of mode hopping when used in the interferometers as they are. This can make it difficult to count changes of interference phases.
The gage interferometers using the commercially available semiconductor lasers as a light source are normally provided with various countermeasures for stabilizing the wavelength to fix the oscillation wavelength in a single mode. One method involves the addition of a temperature control function of a laser by a Peltier device or a heater, but the size, etc., thereof can pose a problem. There is another mode-stabilizing method of returning the laser light to the semiconductor laser by an external mirror or the like, but adjustment and assembly steps become complicated. Another conceivable technique is to form a Bragg diffraction grating structure in the semiconductor laser element itself to raise the wavelength selectivity of the element. However, since the commercially available elements are those for communication fields, they cannot be applied to the gage interferometers as they are.
Meanwhile, the applicant or assignee of the application filed an application to describe an encoder based on the principle of grating interference using a multi-mode semiconductor laser, and the reason for employment thereof is that it is free of the mode hopping phenomenon. Since the multi-mode semiconductor lasers have low coherency, it is necessary to adjust the optical path difference between two coherent beams to near zero as a known technique. However, since the encoder of the grating interference method is based on the principle of varying the phase of the wavefront by moving the diffraction grating, optical path lengths do not vary after once adjusted to an equal length, and thus stable signals to a certain extent are obtained even with use of the multi-mode semiconductor laser.
However, where the multi-mode semiconductor laser is applied to an optical system that varies the optical path lengths, the coherence length is desirably as long as possible in order to increase the measurement range and obtain signals on a stable basis. Even in the case of optical systems in which the optical path lengths are fixed in terms of the principle, such as grating interferometers, laser Doppler velocimeters, and so on, the longer coherence length is rather desirable in order to stabilize the signals in consideration of variations in the optical lengths due to secular change or the like.
In view of these conventional examples, an object of the present invention is to provide an interferometer capable of yielding interference signals on a stable basis, a displacement measuring apparatus, and an information recording or/and reproducing apparatus using it.
The other objects of the present invention will become apparent in the description of embodiments which follows.
An interferometer according to the invention includes a semiconductor laser which oscillates in multiple modes, an optical member for providing a beam from the semiconductor laser with a substantial optical path difference between optical paths partially in one beam, and an interference optical system for causing interference, using the beam having traveled via the optical member.
A displacement measuring apparatus according to the invention includes a semiconductor laser which oscillates in multiple modes, an optical member for providing a beam from the semiconductor laser with a substantial optical path difference between optical paths partially in one beam, and an interference optical system for causing interference, using the beam having traveled via the optical member. The interference optical system comprises a measurement path and a reference path, and multiplexes two beams having traveled respectively in the two paths to form an interference beam. At least the measurement path runs via a surface to be measured. In addition, the apparatus includes a detector for receiving the interference beam, wherein displacement information of the surface to be measured is obtained from output of the detector.
An information recording or/and reproducing apparatus according to the invention includes a semiconductor laser which oscillates in multiple modes, an optical member for providing a beam from the semiconductor laser with a substantial optical path difference between optical paths partially in one beam, and an interference optical system for causing interference, using the beam having traveled via the optical member. The interference optical system includes a measurement path and a reference path and multiplexes two beams having traveled respectively in the two paths to form an interference beam. At least the measurement path runs via a surface displaced together with a recording or/and reproducing portion which makes access for information recording or/and reproduction to an information recording medium. In addition, the apparatus includes a detector for receiving said interference beam. Displacement information of the recording or/and reproducing portion is obtained from output of the detector, and a control system for carrying out control of relative position of the recording or/and reproducing portion of the information recording medium, based on the displacement information.
A servo track writer according to the invention includes a semiconductor laser which oscillates in multiple modes, a Fresnel zone plate for converging a beam from the semiconductor laser, and an interference optical system for causing interference, using the beam having traveled via the Fresnel zone plate. The interference optical system includes a measurement path and a reference path, and multiplexes two beams having traveled respectively in the two paths to form an interference beam, wherein the beam traveling via the measurement path is reflected by a surface of a head arm supporting a magnetic head in a hard-disk drive unit and the beam traveling via the reference path is reflected by a reference surface. The system also includes a detector for receiving the interference beam, wherein displacement information of the head arm is obtained from output of the detector, and a control system for carrying out control of relative position of the head arm to a hard disk, based on the displacement information, wherein a servo track signal is recorded on the hard disk by the magnetic head on the head arm under the control of relative position.